BB, your link to the exhaust gas test method underscores my point about the difficulty in performing said test. Even though you may be holding the probe above the coolant filler, we all know that a small geyser can suddenly erupt from the cooling system (especially since there may be combustion pressures present) and immerse the probe in liquid (ask me how I know). Hold the probe too far away and you negate the efficacy of your test, too close and you suck fluid.
The shortcoming of the 100psi test is mentioned in the link also. If the head gasket will leak bubbles at 100psi, it will be BLOWING OUT at combustion pressures.

Whenever I am replacing an engine due to overheating, I always make sure it is blown. The first step I take is fill it up with water, leave the cap off, and spin it over. 99% of the time I have a geyser of water shooting out. That always confirms that the engine is blown. I am pretty sure that you have a hard time keeping the probe dry while there is water shooting out.